1. Field of the Invention
This invention relates to optical processing apparatus and systems, and to methods of processing optical communication signals, and particularly to systems, methods and apparatus for manipulating wavelength division multiplexed optical signals.
2. Related Art
Optical fibres are an extremely efficient transmission medium. Presently, the capacity of optical fibre communications systems is limited by certain factors, including the way in which the optical fibre bandwidth is utilised, and the opto-electronic components required to control certain optical communication processing functions.
The first factor, that of bandwidth use, is generally addressed by the use of various multiplexing techniques, for example wavelength division multiplexing (WDM) or optical time division multiplexing (OTDM).
The second factor has been extensively investigated over the past six or seven years, the results being demonstrations of all-optical processing functions in optical fibres and semiconductor optical devices. An optical fibre communications network incorporating only all-optical processing functions would potentially provide communications capacity far beyond that which is currently available in optical fibre communications networks incorporating very much slower opto-electronic processing functions.
In terms of bandwidth usage, WDM networks have received considerable attention in recent years, and are likely to provide optical routing in, for example, a metropolitan or national network, where a large node density makes the simple passive demultiplexing (wavelength filtering) associated with WDM attractive. However, the combination of dispersion and fibre non-linearity potentially restricts the size of WDM networks, or the ability to expand WDM networks, if traditional signalling formats are employed. Therefore, presently OTDM is more likely to find application over wider geographical areas, with a smaller number of higher capacity optical switches, since a single wavelength, multiplexed channel system such as OTDM is not so susceptible to the detrimental effects non-linearity and dispersion as a WDM system, particularly when soliton transmission effects are employed to balance non-linearity against dispersion. Furthermore, gain flatness equalisation or pre-emphasis techniques are not an important consideration for single wavelength OTDM systems, whereas such techniques would be an important aspect of the design of a corresponding WDM system, considerably simplifying amplifier (or power) management.
Recognising the problems of scalability associated with WDM communications networks, but at the same time appreciating that WDM has many advantages, for example simple passive demultiplexing, the applicants have considered that in future there might be a need for an all-optical communications network which is potentially able to deal with WDM traffic (eg on a local scale), OTDM traffic (eg on international trunk routes), and soliton traffic (eg on information super-highways). To be effective, such an optical network would also need to be able to convert between any two of the traffic formats employed, otherwise universal interconnection to, and information interchange across, the network would be restricted.
Presently, generation and transmission of WDM, OTDM and soliton optical signalling formats is known and has been widely reported. Also, Lacey, et al. "All optical WDM to TDM transmultiplexer", Electronics Letters, Sep. 15, 1994, pp 1612-1613, proposes WDM to TDM conversion firstly by splitting the WDM signal into its constituent channels using wavelength selective filters and mixing each channel with a clock pulse in separate respective optical amplifiers. This has the effect that gain compression causes wavelength conversion and reduces the width of the WDM data pulses. Then, each channel is delayed by separate respective optical delay lines having different delays, and finally all the channels are remultiplexed using an optical coupler. Bigo, et al, "Bit-rate enhancement through optical NRZ-to-RZ conversion and passive time-division multiplexing for soliton transmission systems", Electronics Letters 1994, vol. 30, pp 984-985, proposes using an optical loop mirror as an AND gate for an NRZ data signal and a clock signal to provide NRZ to RZ conversion. Further, Bigo et al. proposes multiplexing a plurality of these AND outputs to provide bit-rate enhancement (TDM).
Throughout the present description, the terms "square" and "pulsed", with respect to wave forms, are intended to be synonymous and interchangeable with "NRZ" and "RZ" respectively.